Radio frequency (RF) filters are key components in any wireless system and as these systems continue to be miniaturized, the pressure on filter technology to shrink as well without compromising performance continues. Handheld systems and their associated volumes have generated strong interest in filter technologies that show promise for lower cost and smaller size. With energy-hungry applications proliferating in modern handsets, low insertion losses extending talk time and battery life also become highly valuable. Band-pass filters that incorporate one or more film bulk acoustic wave (BAW) resonators, known for their high quality factors which directly translate into smaller in-band insertion loss, has recently emerged as an advantageous alternative to filters technology based on both surface acoustic wave (SAW) resonators and ceramic resonators. BAW filters can be classified into two basic categories depending on how the resonators are connected: electrically connected (ladder, lattice or similar configurations) and acoustically coupled (stacked crystal filters (SCF) and coupled resonator filters (CRF)). Acoustically coupled resonators can achieve higher rejection at the far stop-band and wider bandwidth than electrically connected resonators. A CRF can be considered as an extension of a SCF where the acoustic coupling between resonators is less than the one by direct contact in SCF and controlled to achieve higher bandwidths. As shown in FIG. 1, the basic structure of a CRF uses two single BAW resonators vertically arranged one on top of the other and decoupled by means of an acoustic decoupler that can be a single layer or several passive layers of materials having different acoustic impedance. Since the single resonators are replaced by pairs of stacked identical resonators, the number of individual resonators in the CRF filter is small and total area required to implement the filter is reduced, thereby savings in die size and manufacturing costs are realized.
As CMOS for RF transceiver in mobile handsets goes to smaller nodes, the power supply voltage must also shrink. Going differential allows for the same voltage swing, but greatly reduces any common mode signal. Major mobile phone standards like W-CDMA (Wideband Code Division Multiple Access) and GPS (Global Positioning System) front-end module are pushing the need for filter devices featuring single-to-balanced conversion because the LNAs are integrated into RF transceiver ICs and typically have balanced inputs and high impedance. In order to establish BAW devices as a mainstream filter technology the capability for mode conversion become mandatory. BAW filters with a lattice or ladder topology can only provide either single-ended or balanced filters. Therefore, additional efforts are needed, for example, via external baluns, to have the mode conversion option. However, BAW filters with baluns lose some insertion loss and require additional cost and space on the board. CRFs offer complete galvanic isolation between input and output and thus enable to offer BAW filters with mode-conversion (single-ended to balanced) as well as impedance transformation.
The basic structure of BAW resonator and CRF stack is usually suspended as a membrane over an air cavity defined in or on the substrate to completely prevent the acoustic wave generated in the acoustic stack from propagating into the substrate. For example, first, a cavity is etched within the substrate and a layer of sacrificial material is deposited on the surface of the wafer with a thickness sufficient to fill the cavities. The surface of the wafer is then planarized to leave the cavity filled with the sacrificial material. After that, the acoustic resonator stack is fabricated on top of sacrificial layer. Lastly, sacrificial material is etched away from the cavity through exposed via holes to form the air gap underneath CRF.
In manufacturing of BAW filters, the various layers in the resonator device are sequentially formed by thin film deposition and the resonant frequencies in BAW resonators essentially depend on thicknesses of the individual layers (electrode layers, piezoelectric layer, etc.) in the stack. To meet the stringent specification for filtering use in mobile phones, the resonant frequency of the device usually has to be controlled to within a 0.1% tolerance. This means that, if no tuning is used, the thickness of each layer in the device must be controlled in a similar way. It is known that, however, the deposition of thin-film layers by the methods typically used in thin film technology, for example, physical vapor deposition (PVD), chemical vapor deposition (CVD), e-beam evaporation, etc., is extremely difficult to yield a thickness uniformity within such a tight tolerance within the substrate and from substrate to substrate.
Compared to electrically connected BAW filter with single piezoelectric layer, trimming of CRF is much more demanding. A CRF typically includes two piezoelectric layers, four electrodes and one decoupling layer. The first piezoelectric layer is arranged between a first bottom electrode and a first top electrode, a second piezoelectric layer is arranged between a second bottom electrode and a second top electrode. The decoupling layer located between the first top electrode and second bottom electrode controls the degree of acoustic energy coupling between the lower and upper resonator (lower refers typically to the direction towards substrate). Based on the state of the art accuracy (about 0.5% thickness standard deviation) of thin film deposition processes, it is currently not possible to produce CRFs with a reasonable manufacturing yield, if only relying on deposition accuracy. Furthermore, it has been observed that the bandwidth of filter is determined by the acoustic decoupler and the frequency position of lower resonator (arranged between the air cavity and the upper resonator) determines the passband frequency of the filter within very tight limits. The upper resonator has to be tuned appropriately to the frequency of the lower resonator and the trimming of upper resonator alone cannot tune the filter central frequency or bandwidth.
Thus, it is advantageous and desirable to provide a robust and inexpensive manufacturing method to solve the problem associated with thickness non-uniformity in the fabrication of coupled resonator filter devices in particular, on large substrates or wafers.